In view of numerous factors such as higher energy prices and environmental concerns, the production of value-added gaseous products from lower-fuel-value carbonaceous feedstocks, such as petroleum coke and coal, is receiving renewed attention. The catalytic gasification of such materials to produce methane and other value-added gases is disclosed, for example, in U.S. Pat. Nos. 3,828,474, 3,998,607, 4,057,512, 4,092,125, 4,094,650, 4,204,843, 4,468,231, 4,500,323, 4,541,841, 4,551,155, 4,558,027, 4,606,105, 4,617,027, 4,609,456, 5,017,282, 5,055,181, 6,187,465, 6,790,430, 6,894,183, 6,955,695, US2003/0167961A1, US2006/0265953A1, US2007/000177A1, US2007/083072A1, US2007/0277437A1 and GB 1599932.
Gasification of a carbonaceous material, such as coal or petroleum coke, can be catalyzed by loading the carbonaceous material with a catalyst comprising an alkali metal source. US2007/0000177A1 and US2007/0083072A1, both incorporated herein by reference, disclose the alkali-metal-catalyzed gasification of carbonaceous materials. Lower-fuel-value carbon sources, such as coal, typically contain quantities of inorganic matter, including compounds of silicon, aluminum, calcium, iron, vanadium, sulfur, and the like. This inorganic content is referred to as ash. Silica and alumina are especially common ash components. At temperatures above 500-600° C., alkali metal compounds can react with the alumina and silica to form alkali metal aluminosilicates. As an aluminosilicate, the alkali metal compound is substantially insoluble in water and has little effectiveness as a gasification catalyst.
At typical catalytic gasification temperatures, most components of ash are not gasified, and thus build up with other compounds in the gasification reactor as a solid residue referred to as char. For catalytic gasification, char generally includes ash, unconverted carbonaceous material, and alkali metal compounds (from the catalyst). The char must be periodically withdrawn from the reactor through a solid purge. The char may contain substantial quantities of alkali metal compounds. The alkali metal compounds may exist in the char as soluble species, such as potassium carbonate, but may also exist as insoluble species, such as potassium aluminosilicate (e.g., kaliophilite). It is desirable to recover the soluble and the insoluble alkali metal compounds from the solid purge for subsequent reuse as a gasification catalyst. A need remains for efficient processes for recovering soluble and insoluble alkali metal compounds from char. Such processes should effect substantial recovery of alkali metal compounds from the char, minimize the complexity of the processing steps, reduce the use of consumable raw materials, and generate few waste products that require disposal.